Soils 497A Urban Soils

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Soils 497AUrban Soils

• Landscape Stability 3
• Water Flow

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Brady and Weil, 2002
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Types of Erosion

• Water Erosion
• Wind Erosion
• Ice Erosion

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Water Erosion

• There are three types of water erosion. In the
  order of intensity, sheet, rill, gully.
• On the average there is approximately 4 to 10
  tons of soil eroded from an acre of a typical 3
  percent sloped agricultural field each year.
• Soil erosion from water is the major cause of
  aquatic polution.

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Sheet Erosion
Surface runoff forms when the rainfall intensity
of a storm exceeds the infiltration capacity of
the soil. Sheet erosion is caused by the
unconfined flow of water running across the
surface. The effects of sheet erosion are often
hard to distinguish because such thin layers of
soil are being removed. It isn't until several
years later that significant degradation is
perceived.
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Rill Erosion
Rill erosion is caused by water concentrating
into innumerable, closely-spaced small channels.
Left unchecked, rills can cut vertically and
horizontally and when joined, causing gullies.
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Gully Erosion
Gullies are steep-sided trenches formed by the
coalescence of many rills. Once started they are
difficult to stop. 
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Wind Erosion

• There are three stages to wind erosion. Particle
  lift, transport, and collision. Then the process
  starts all over again.
• Dust can be transported between continents.
• Low moisture soils with fine textures are the
  most susceptible to wind erosion.

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A blustery day in Iraq
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David Kohake3 January, 2006
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Erosion by Ice

• There are two ways ice can influence erosion. One
  is the effects of Glaciers, and the more common
  in temperate areas is the freeze-thawing of rock
  and soils.
• The water percolates into cracks and pores then
  freezes causing the frozen water to expand. This
  then can cause instability within the rock or
  soil.

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On the left is a photograph of Muir Glacier taken
on August 13, 1941, by glaciologist William O.
Field on the right, a photograph taken from the
same vantage on August 31, 2004, by geologist
Bruce F. Molnia of the United States Geological
Survey (USGS).
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Greg Epperson
John Harvey
John Harvey
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Runoff

• Calculations of Runoff are important to your soil
  stability

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Runoff Rate Calculation
Q CIA
Where Q Peak Runoff C Runoff
Coefficient I Rainfall Intensity A
Drainage Area
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2.75 inches within 24 hours for 2yr storm
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3.5 inches within 24 hours for 5 year storm
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4.75 inches within 24 hours for 25 year storm
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6.8 inches within 24 hours for 100 yr storm
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Runoff Coefficient
Cw (C1 x A1) (C2 x A2) (Cn x An)
A (total)
Where Cw Weighted Average C Runoff
Coefficient A Area of Individual Landuse
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Rainfall Intensity

• Typically measured in inches of rainfall within
  1, 2, 5, 10 year intensity storms.
• Available for the entire Continental U.S. in the
  Technical Paper 40 from the U.S. Commerce
  Department. OR http//www.ncdc.noaa.gov/oa/documen
  tlibrary/rainfall.html
• These tables are what everyone uses for
  determination of the rainfall intensity.

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Time of Concentration

• Is the time that runoff takes to traverse the
  distance between the hydraulically longest
  watershed flow path
• It is based on the length of the flow path,
  surface slope and surface roughness.
• I also takes account of sheet, shallow
  concentrated flow and channel flow.
• The result gives the intensity of rainfall needed
  to input into the equation.

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Time of Concentration
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Rainfall Intensity determination
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Runoff Rate

• After all the items are figured out, the results
  are inputted in the QCIA and presto, the cubic
  ft / second of water running off the site is
  found.
• Of course there are many assumptions made about
  the infiltrative rates of the soils, and the
  effects of the vegetation, but the form is fairly
  accurate.

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Runoff Amounts

• The runoff amount is different than the runoff
  rate.
• The rate is usually in cubic feet per second
  while the amount might be in acre feet, or
  inches per square foot.

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Amount Calculation
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BMPs for ES Control

• Rock Construction Entrances
• Vegetative Filter Strips
• Straw Bales
• Filter Fence
• Rock Filter Outlets
• Silt Fence

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Common Sense BMPs

• Establish vegetation as quickly as possible.
• Reduce the length of slopes
• Stagger Earthmoving Activities so that the
  minimum amount of bare soil is exposed.
• Keep excessive concentrated flow from entering
  the site.
• Reduce the amount of concentrated flow within the
  site.

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Stormwater Management

• Handling On-Site Water during and after
  construction

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Stormwater Management

• There will be a new law and regulations in
  Pennsylvania dealing with Stormwater Management.
• The Plan will enforce the idea to maintain as
  much runoff on the site and redirect it back to
  the aquifers.

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Info Tidbits

• Bank-full stream flow occurs between 1 2 year
  storm event frequencies under pre-developed
  conditions.
• Urbanization causes bank-full stream flow to
  occur more often than 1 year frequency events.
• 80 of annual precipitation infiltrates into the
  soil in pre-development areas.

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PA Stormwater Management Standards

• Do not increase post-development total runoff
  volume for all storm equal to or less than 2
  year/24 hr events
• Do not increase peak runoff for (1, 2, 10, 25,
  and 100 yr) pre-development versus
  post-development.
• Pre-development non-forested pervious areas must
  be considered meadow or equivalent.
• Achieve an 85 reduction in post-development TSS
  and TP loads within the runoff.

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PA Stormwater Manual
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Stormwater BMPs

• Are broken down into two categories, Structural
  and Non-structural.
• Structural BMPs are part of the design and
  require soils knowledge to implement correctly.
• Non-structural BMPs are more of common sense
  approach to mitigating the effects of development.

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PA Stormwater Manual
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PA Stormwater Manual
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